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Within certain limits, TLS server software can choose what kind of cryptography to use when a client connects. These choices can affect security, compatibility, and performance in complex ways. Most of these options are independent of a particular certificate. The Let's Encrypt client tries to provide defaults that we think are most useful to our users.
As described below, the Let's Encrypt client will default to modifying server software's cryptographic settings to keep these up-to-date with what we think are appropriate defaults when new versions of the Let's Encrypt client are installed (for example, by an operating system package manager).
When this feature is implemented, this document will be updated to describe how to disable these automatic changes.
Software that uses cryptography must inevitably make choices about what kind of cryptography to use and how. These choices entail assumptions about how well particular cryptographic mechanisms resist attack, and what trade-offs are available and appropriate. The choices are constrained by compatibility issues (in order to interoperate with other software, an implementation must agree to use cryptographic mechanisms that the other side also supports) and protocol issues (cryptographic mechanisms must be specified in protocols and there must be a way to agree to use them in a particular context).
The best choices for a particular application may change over time in response to new research, new standardization events, changes in computer hardware, and changes in the prevalence of legacy software. Much important research on cryptanalysis and cryptographic vulnerabilities is unpublished because many researchers have been working in the interest of improving some entities' communications security while weakening, or failing to improve, others' security. But important information that improves our understanding of the state of the art is published regularly.
When enabling TLS support in a compatible web server (which is a separate step from obtaining a certificate), Let's Encrypt has the ability to update that web server's TLS configuration. Again, this is different from the cryptographic particulars of the certificate itself; the certificate as of the initial release will be RSA-signed using one of Let's Encrypt's 2048-bit RSA keys, and will describe the subscriber's RSA public key ("subject public key") of at least 2048 bits, which is used for key establishment.
Note that the subscriber's RSA public key can be used in a wide variety of key establishment methods, most of which do not use RSA directly for key exchange, but only for authenticating the server! For example, in DHE and ECDHE key exchanges, the subject public key is just used to sign other parameters for authentication. You do not have to "use RSA" for other purposes just because you're using an RSA key for authentication.
The certificate doesn't specify other cryptographic or ciphersuite particulars; for example, it doesn't say whether or not parties should use a particular symmetric algorithm like 3DES, or what cipher modes they should use. All of these details are negotiated between client and server independent of the content of the ciphersuite. The Let's Encrypt project hopes to provide useful defaults that reflect good security choices with respect to the publicly-known state of the art. However, the Let's Encrypt certificate authority does not dictate end-users' security policy, and any site is welcome to change its preferences in accordance with its own policy or its administrators' preferences, and use different cryptographic mechanisms or parameters, or a different priority order, than the defaults provided by the Let's Encrypt client.
If you don't use the Let's Encrypt client to configure your server directly, because the client doesn't integrate with your server software or because you chose not to use this integration, then the cryptographic defaults haven't been modified, and the cryptography chosen by the server will still be whatever the default for your software was. For example, if you obtain a certificate using standalone mode and then manually install it in an IMAP or LDAP server, your cryptographic settings will not be modified by the client in any way.
Initially, the Let's Encrypt client will configure users' servers to use the cryptographic defaults recommended by the Mozilla project. These settings are well-reasoned recommendations that carefully consider client software compatibility. They are described at
https://wiki.mozilla.org/Security/Server_Side_TLS
and the version implemented by the Let's Encrypt client will be the version that was most current as of the release date of each client version. Mozilla offers three separate sets of cryptographic options, which trade off security and compatibility differently. These are referred to as as the "Modern", "Intermediate", and "Old" configurations (in order from most secure to least secure, and least-backwards compatible to most-backwards compatible). The client will follow the Mozilla defaults for the Intermediate configuration by default, at least with regards to ciphersuites and TLS versions. Mozilla's web site describes which client software will be compatible with each configuration. You can also use the Qualys SSL Labs site, which the Let's Encrypt software will suggest when installing a certificate, to test your server and see whether it will be compatible with particular software versions.
It will be possible to ask the Let's Encrypt client to instead apply (and track) Modern or Old configurations.
The Let's Encrypt project expects to follow the Mozilla recommendations
in the future as those recommendations are updated. (For example, some
users have proposed prioritizing a new ciphersuite known as 0xcc13
which uses the ChaCha and Poly1305 algorithms, and which is already
implemented by the Chrome browser. Mozilla has delayed recommending
0xcc13 over compatibility and standardization concerns, but is likely
to recommend it in the future once these concerns have been addressed. At
that point, the Let's Encrypt client would likely follow the Mozilla
recommendations and favor the use of this ciphersuite as well.)
The Let's Encrypt project may deviate from the Mozilla recommendations in the future if good cause is shown and we believe our users' priorities would be well-served by doing so. In general, please address relevant proposals for changing priorities to the Mozilla security team first, before asking the Let's Encrypt project to change the client's priorities. The Mozilla security team is likely to have more resources and expertise to bring to bear on evaluating reasons why its recommendations should be updated.
The Let's Encrpyt project will entertain proposals to create a very small number of alternative configurations (apart from Modern, Intermediate, and Old) that there's reason to believe would be widely used by sysadmins; this would usually be a preferable course to modifying an existing configuration. For example, if many sysadmins want their servers configured to track a different expert recommendation, Let's Encrypt could add an option to do so.
In the course of considering how to handle this issue, we received recommendations with sources of expert guidance on ciphersuites and other cryptographic parameters. We're grateful to everyone who contributed suggestions. The recommendations we received are available at
https://github.com/letsencrypt/letsencrypt/wiki/Ciphersuite-guidance
Let's Encrypt client users are welcome to review these authorities to better inform their own cryptographic parameter choices. We also welcome suggestions of other resources to add to this list. Please keep in mind that different recommendations may reflect different priorities or evaluations of trade-offs, especially related to compatibility!
This will probably look something like
..code-block: shell
letsencrypt --cipher-recommendations mozilla-secure letsencrypt --cipher-recommendations mozilla-intermediate letsencrypt --cipher-recommendations mozilla-old
to track Mozilla's Secure, Intermediate, or Old recommendations, and
..code-block: shell
letsencrypt --update-ciphers on
to enable updating ciphers with each new Let's Encrypt client release, or
..code-block: shell
letsencrypt --update-ciphers off
to disable automatic configuration updates. These features have not yet been implemented and this syntax may change then they are implemented.
The status of this feature is tracked as part of issue #1123 in our bug tracker.
Prior to implementation of #1123, the client does not actually modify ciphersuites (this is intended to be implemented as a "configuration enhancement", but the only configuration enhancement implemented so far is redirecting HTTP requests to HTTPS in web servers, the "redirect" enhancement). The changes here would probably be either a new "ciphersuite" enhancement in each plugin that provides an installer, or a family of enhancements, one per selectable ciphersuite configuration.